"This has major implications for how to treat patients with severe asthma and could also substantially reduce the use of NHS resources."

Immune response

The team's results imply that steroid treatment works, at least in part, by inducing the T-cells of the immune system to synthesise a secreted signalling molecule, called IL-10.

This molecule can inhibit the immune responses that cause the symptoms of allergic and asthmatic disease.

Unlike T-cells from healthy individuals, or patients that respond to steroids, T-cells taken from patients who are steroid resistant do not produce IL-10 when cultured in vitro with the steroid, dexamethasone.

However, the researchers found that when vitamin D3 was added to the culture medium along with dexamethasone, this defect was reversed.

The researchers gave daily vitamin D3 supplements to people with asthma who were unresponsive to steroids for seven days.

Blood tests showed the patients' T-cells were more responsive to dexamethasone after they had taken the supplement.

Further work needed

Lead researcher Dr Catherine Hawrylowicz said: "At the moment we only have a preliminary experimental observation.

"We now need to test the benefits of this treatment in the clinic, and we are currently putting a proposal together to carry out this work.

"Interestingly, vitamin D3 is at present occasionally administered to patients with severe asthma to help prevent steroid-induced osteoporosis.

"Our studies suggest that there is an additional potential benefit to this treatment."

Dr Lyn Smurthwaite, research development manager at Asthma UK, said: "Two-point-six million people in the UK have severe asthma symptoms, many of whom have restricted treatment options available to them as they do not respond to conventional steroid therapy.

"This research opens up a potentially important new avenue for developing treatments for people with difficult to control asthma."